Numerous electrochromic processes have been proposed and described for modulating light in transmission or reflection in order to display signals and images. These processes provide displays, screens, mirrors or other objects which, totally or partially, reflect or transmit light according to instructions transmitted to them by electrical means.
These electrochromic processes function in accordance with the laws of reversible electrolysis and use the reversible change in color and/or of optical density obtained by electrochemical oxydo-reduction of a so-called "electrochromic" material, the oxidized form and the reduced form of which are of different colors and/or optical densities.
Basically, an elementary light modulating cell, which functions according to an electrochromic process, comprises two electrodes separated by an electrolytic medium consisting of one or several layers. One of the electrodes at least (in the case of a reflection function) should be transparent. Various devices well known to those of ordinary skill in the art protect the electrodes, transmit the current, protect the layer(s) of the electrolyte, and provide the geometric shape of the cell. French patent FR-B-2 618 567 describes such a device for the light modulating.
Electrochromic light modulation processes afford a group of characteristics which may be advantageous for certain applications and which have often been cited. In particular, the following advantages exist:
the possibility of a memory with an open circuit; PA1 a low control voltage; PA1 a fairly large tolerance in the distance between electrodes; PA1 energy consumption limited to that necessary to cause a change in the state of the electrolytic medium and which may therefore be reduced even further for certain uses. PA1 a first substrate, transparent or substantially transparent, having at least one working electrode, in a thin layer, transparent or substantially transparent and being electronically conductive. PA1 A second substrate is placed at a distance transversely in relation to the first substrate and has at least one electronically conductive counter-electrode. The second substrate and the associated counter-electrode(s) may be transparent or substantially transparent. PA1 At least one layer of electrolytic material is between the working electrode(s) and counter-electrode(s), the composition of which makes it possible to ensure electrochromism. PA1 An electric current is supplied to the ends of the working electrode(s) and counter-electrode(s), which may be completed by means of supplying electric current within the working electrode(s) and/or counter-electrode(s) if the latter is (or are) transparent or substantially transparent. It is possible to provide means of addressing in the case of a juxtaposition of cells in order to control them selectively. PA1 Means for protecting the working electrodes and counter-electrodes and means for protecting the layer(s) of electrolytic material are also provided. PA1 a work table perforated with holes arranged according to the network of the grid. PA1 A plate is arranged under the work table, parallel to the latter, bearing needles with sections corresponding approximately to the dimensions of a mesh of the grid. The plate is raised and lowered respectively in order to project the needles through the holes in the work table, for the purpose of positioning the grid on the latter, and to remove them, for the purpose of applying, on to the grid positioned in this way the transparent substrate on which the electrodes or counter-electrodes have previously been deposited. PA1 Means for supporting the transparent substrate is provided which is capable of lowering it onto the grid in position on the work table and capable of raising it once it has been made integral with the grid. PA1 Means of control is fitted on the work table and/or the means of support, in order to ensure an exact positioning of the conductive wires of the grid along the conductive lines of the network of electrodes and counter-electrodes. PA1 Means for applying heat and pressure to the grid/substrate unit is provided in the case where the conductive wires are provided with a thermoplastic sheath.
As regards display applications, it is possible, for some of these processes, to obtain an excellent contrast, even in side vision with a high angle. Contrary to the situation with some processes which operate by transmission of polarized light, and those which have poor visibility in daylight, this process is very suitable for electrochromic display units-operating by reflection, even when they are placed outside in strong sunlight.
The practical achievement of these advantages is subject, however, to the solution of certain operating difficulties which the different processes proposed are intended to reduce.
Thus, for example, the need to avoid deterioration, indeed the disappearance of the electrolytic medium, resulting in the degradation and the inadequate lifetime of the cell, is at the basis of a large number of proposals regarding the choice of the electrolytic layer(s), and the practical creation of rigorously sealed cells. Other problems concern the practical difficulty of installing the components of the electrolytic medium. For example, it is very difficult, if not impossible, to obtain, with an electrolytic liquid, electrochromic cells of very small dimensions. The following procedure is used at present, in particular in the case of small pixel screens or displays obtained from transparent "columns" and "lines" on a non-transparent base:
The transparent electrode is created from a conductive transparent layer which is itself deposited onto a transparent substrate, such as glass or a transparent plastic material. This transparent conductive layer is made up of one or several metals or oxides, such as, for example, gold, silver, tin oxide (TO), indium tin oxide (ITO), zinc oxide and cadmium stannate etc, as mentioned in the literature. In order to remain transparent, these layers must be very thin (some hundredths to some thousandths of an Angstrom). Various very well known processes are therefore used very economically to deposit these materials, such as spraying powders or liquids which decompose on to the hot substrate, chemical vapor deposition (CVD), depositions under vacuum, or even chemical precipitations. Nevertheless, the slight thickness required does not make it possible to obtain a high level of conductivity in the layer, the conductivity being proportional to the thickness for a given metal or oxide and for a given deposition process. A compromise must be made between transparency and thickness.
Furthermore, these transparent electrodes are created in the form of fine lines insulated from each other (which will form the "columns" of the screen or the display), either by depositing a continuous transparent conductive layer over the whole surface of the transparent substrate, and then "engraving" it by processes currently used in photolithography, or by covering the surface of the transparent substrate with an appropriate mask and then depositing the conductive metal or oxide, the deposition being effected solely on the areas of the columns left accessible by the mask, which is subsequently removed.
On the basis of known techniques, it is therefore possible to create transparent electrodes of the desired fineness, but one problem still persists when the screen is to have large dimensions because the electrochromic process imposes a slight ohmic loss and a relatively high current. The resistance of the column which transmits the control is then too high and it is not sufficient to supply the current to the two ends of the column. It is advisable to provide additional supplies of current to the transparent electrode between the two ends of the column, i.e. into the interior of the screen itself. It may be mentioned in passing that this problem also arises with screens or displays with small transparent pixels, i.e. in which the "lines" are obtained in the same way as the columns, in which case it may also be necessary to supply additional current to lines of large dimensions. A similar problem is presented in the case of displays, or more generally, in light modulators, be they non-transparent or transparent. The light modulators are comprised of large dimension pixels, the form of which corresponds, for example, to that of letters in the case of non-transparent displays, or to that of the modulator itself, for example, in the case of glass panes which can be darkened electrically. In this case, the objective is to distribute the current into the interior of the large dimension pixel.
Various solutions have been proposed to solve the problems which have just been described, but they are all impractical.
Some consist of forming a very conductive "counter-column" on the back of the screen and of making the counter-column communicate with the ITO column, for example, at certain points by means of "nails" or "rivets" crossing the film which supports the "lines", then crossing the electrolytic layer(s). This operation is delicate and costly, as the "nail", of very small dimensions, must be placed in position with great precision. There may be several per column and there may be several hundred columns.
The "nail" must be insulated in the part which crosses the electrolyte layer(s). In addition, it must, however, be in contact with the transparent conductive layer, made of a non-corrodible conductive material (or protected against corrosion also at the point of contact). It must finally be in contact with the counter-column, which requires soldering or adhesion operations of a precise and delicate nature.
Another solution consists of fixing, along the column, a conductive wire so fine that it is almost invisible, to supply the current, in contact with the conductive layer, and non-corrodible or protected against corrosion. It is indeed known that the visibility of a line is practically nil if it is seen at an angle ranging from some tens of seconds to one minute of an arc. This very fine wire should be placed with great precision from the top to the bottom of the column.
In these small pixel screens and displays, the creation of the non-transparent electrodes (also referred in the present description by the term "counter electrodes") is simpler in principle as it does not have to conform to a thickness limit imposed by the transparency. When large pixels are to be created, the use of counter-electrode materials available in the form of very thin sheets and layers has been proposed in previous technology.
The most interesting electrochromic processes use, however, electrolytic materials which are more or less corrosive, sometimes which are very corrosive, and thus the choice of material used for manufacturing the counter-electrode in these processes is a matter of some importance. It must both conduct the current and not be liable to be corroded by the electrolyte at rest or in operation. To this effect pliable sheets of pyrolytic graphite, a plastic material loaded with particles of carbon or metal, certain conductive pastes for serigraphy, graphite or carbon fabrics or cloths have been suggested.
When the creation of a screen with numerous very small pixels is desired, the counter-electrodes which will form the "lines" of the screen must be made in the form of fine strips insulated from each other. For this it has been proposed that a conductive sheet of the type mentioned in the previous paragraph should be cut into fine strips which must then be stuck alongside each other on a suitable support, separated by a narrow interval. If the sheet is made, for example, from conductive metal covered with a protective varnish, the cut edge must also be provided after cutting with a protective varnish if one wishes to prevent the electrolytic medium (media) from attacking the counter-electrode at the cut edge. One can imagine that a process of this kind would be expensive, and almost impracticable due to a certain degree of fineness required for the "lines".
This is why other methods have been proposed, based on the deposition of the material making up the counter-electrode by a printing process, such as that of serigraphy or ink jet, but these processes themselves encounter certain difficulties which make the creation of very fine and non-corrodible conductive counter-electrodes difficult, if not impossible. Thus, for example, the precision-which can be achieved by serigraphy does not lend itself well to the deposition of several layers by successive coating, some of them--such as silver paste--, intended to ensure the conductivity of the line, others,--such as ink filled with graphite--, intended to ensure the protection of this line against corrosion. The ink jet, another process which might produce the desired fineness, does not tolerate well chemical compositions containing particles or micelles which are to be deposited, these particles or micelles blocking the distribution tube or obstructing the splitting-up of the jet into sufficiently fine drops.